This invention relates to improved sealing arrangements for use in various types of processing apparatus with such sealing arrangements being particularly, but not exclusively, suited for use in cannister multicyclone arrangements.
In recent years the so-called cannister arrangement for hydrocyclones has been widely adopted by the pulp and paper industry, particularly in the case of small hydrocyclones having an effective diameter of up to about 125 mm. These arrangements, which are commonly used in the cleaning of pulp slurry, i.e. the removal of both heavy and light impurities therefrom, allows for a very neat and compact layout of the entire cleaning system thus reducing the number of pipes and piping connections and, in general, allowing for very good housekeeping. These pulp stock cleaning systems usually consist of from 2 to about 5 stages and these cannisters which may constitute part but usually the entire stage of the system may serve to contain anywhere from 3 to about 400 hydrocyclones. There are at the present time approximately 10 commercial manufacturers of cannister multicyclone equipment including such important manufactureres as Noss, Celleco, Albia, and Black Clawson.
Two basic designs of cannister multicyclone apparatus are known in the art.
In the first group may be classified cannisters in which the partitions separating the accept, feed and reject chambers are concentrically arranged and cylindrical in shape thus being subjected substantially to hoop stresses only. The partitions are hence made of relatively thin steel of a thickness from about 1.5 to 3.5 mm. In practice the individual hydrocyclones are inserted radially from the outside of the cannister through apertures made in the partitions. The outermost cylindrical shell or envelope may be, during installation, located in a raised position with such outermost shell being thereafter lowered downwardly so as to enclose the entire system; alternatively, this outermost shell may be stationary with openings for inserting and removing each cleaner, each cleaner, during operation, being sealingly covered by a special cap. In both cases, the 2 or 3 concentrically arranged partitions (depending on whether an elutriating system is provided) are provided with aligned sets of perforations or apertures which serve to nest the individual hydrocyclones. Groups of such hydrocyclones, disposed in radially arranged arrays are spaced apart from each other along the vertical axis of the cannister.
The apertures which serve to nest the hydrocyclones must be made in the form of so-called "pulled out" or "pushed in" holes or apertures. This technology involves precisely marking the centers of such apertures and then cutting out small holes and then inserting into these holes a metal pusher or puller and then pressing the same radially to form a cylindrical collar. Thereafter the uneven and cracked edges at the ends of the collars are trimmed. Later on, during assembly of the cannister system, the "pulled out" or "pushed in" holes in the second, third and sometimes fourth partitions, depending upon the cannister design, must be properly aligned to permit subsequent installation of the hydrocyclones. This can be a very formidable task considering that sufficient of these holes must be provided to accommodate, in certain designs, up to approximately 400 hydrocyclones. It has to be kept in mind that the alignment and concentricity of all of the collars as thus formed is very critical. The hydrocyclone body is usually made of a somewhat elastic material. It cannot be soft as buckling or collapse of the hydrocyclone inwardly under feed pressures that are higher than the pressure inside the hydrocyclone must be avoided. Still another problem with the prior art systems is to make the "pulled out" or "pushed in" collars in relatively high tolerances thereby to accommodate the various types of seals used between the hydrocyclones and the partition walls.
Three basic types of seals are in common use. The first type consists of a plurality of elastic annular ribs disposed in series along the axis of the cleaner. The cleaner is made of a resilient material such as polyurethane, hard rubber or vinyl and the sealing ribs are integrally molded with the body of the hydrocyclone. Two or three sets of such ribs, depending upon the number of partitions, are spaced apart along the hydrocyclone with the diameters of same increasing from the small end of the hydrocyclone to the large end. At the time of installation these ribs must fit inside the collars formed in the partitions with locational transition fit thereby to provide for relative ease of installation while at the same time providing for adequate sealing engagement while later on accommodating for some expansion of the hydrocyclone due to moisture pickup by the elastomer from which the hydrocyclone is made.
The second type of seal is known as a "labyrinth" type of seal and this type is used when the hydrocyclone is made of a relatively hard compound such as fiber reinforced nylon. In this case the labyrinth seal is made of soft rubber in the shape of a plurality of spaced apart annular ribs. Each labyrinth seal in the form of an annular body is maintained in position on the hydrocyclone by means of a molded tongue which fits in a groove in the outer surface of the cleaner. Radially outward extremities of the annular ribs of the labyrinth seal contact the inner generally cylindrical surface provided by the collar which was previously formed in the associated aperture and, while the labyrinth seal does not serve to prevent all leakage at start up, it has been found that after the grooves fill up with fiber, the seal fulfills its role.
The third main type of sealing arrangement, instead of using a series of annular ribs, uses "O" ring seals cooperating with the collar formed in the partition. Each "O" ring is located in an associated groove formed in the hydrocyclone body and it is of relatively large cross-section.
Reference was made above to the first group of cannister arrangements incorporating relatively thin cylindrical partition walls. The second basic group of cannister designs is structured with flat partitions having aligned sets of apertures therein so that the hydrocyclones can be inserted into them and positioned parallel to each other. Since the partitions are flat they must be of significant thickness to withstand the pressure forces thereon and they are often reinforced by means of spaced rods linking the partitions together. Because of the thickness of the partitions, the "pulled out" or "pushed in" configuration defining the collars cannot be used as in the case of the thin walled partitions. The apertures for these relatively thick partitions must be punched out or machined by drilling or boring. If the standard types of seals described above are to be used, these apertures must be machined with a high degree of accuracy in order to permit the seal to function adequately. This machining and finishing operation gives rise to very substantial costs.
Two basic seal types are commonly used with the above relatively thick partitions. When the wall thickness is of the order of 8 to 13 mm., the seals may be of a design similar to the first two types of seals described above in connection with the "pulled out" collars. In other words the seals employ a plurality of annular labyrinth style ribs but in the case of the relatively thick partitions, the seal ribs must be made considerably smaller so as to have at least two of these ribs in engagement with the cylindrical surface or wall defined by the hole. (It should be noted here that the "pulled out" collar formed in the thin walled partition has an axial length which is very much greater than the axial length of the hole in the wall formed in the relatively thick walled partition.)
The second type of seal for use with the thick walled partitions utilizes a special elastomeric gasket which fits around the perimeter of the aperture. The gasket has a groove on the outside corresponding in width with the thickness of the plate. The gasket also has at least one and usually two radially inwardly directed lips, which lips engage the surface of the hydrocyclone cleaner thereby to provide the necessary sealing effect. Since in the usual cannister arrangement there are two or three partitions with apertures therein of decreasing diameter it is very difficult to position these seals in the apertures before the hydrocyclone cleaner is inserted. There is a danger that the gasket may disintegrate or be lost inside the cannister and retrieval of same may be impossible.
It will be readily appreciated from the above that both these groups of cannister designs give rise to difficulties insofar as sealing arrangements are concerned. In the case of the first group of cannister designs employing relatively thin partition walls and "pulled out" or "pushed in" collars, a substantial degree of expense in involved in the trimming and finishing of the collars and providing same with the relatively high tolerances involved as well as providing for accurate alignment of same during assembly of the cannister. Likewise in the case of the second group of cannister designs employing flat walled relatively thick partitions, substantial costs again are involved in forming the apertures by way of machining operations and finishing same to the relatively high degree of accuracy required. Certain of the seal designs described above also tend to be relatively costly and certain of them give rise to the problems as noted previously.